1. Field of the Invention
This invention relates to processes for the conversion of natural gas to hydrocarbon liquids. More particularly, this invention relates to processes for the conversion of natural gas to hydrocarbon liquids wherein natural gas is first converted to reactive hydrocarbon products and the reactive hydrocarbon products are then reacted further to produce the hydrocarbon liquids.
2. Description of the Related Art
Natural gas typically contains about 60-100 mole percent methane, the balance being primarily heavier alkanes. Alkanes of increasing carbon number are normally present in decreasing amounts. Carbon dioxide, hydrogen sulfide, nitrogen, and other gases may be present in relatively low concentrations.
The conversion of natural gas into hydrocarbon liquids has been a technological goal for many years. This goal has become even more important in recent years as more natural gas has been found in remote locations, where gas pipelines cannot be economically justified. A significant portion of the world reserves of natural gas occurs in such remote regions. While liquefied natural gas (LNG) and methanol projects have long attracted attention by making possible the conversion of natural gas to a liquid, in recent years the advent of large-scale projects based upon Fisher-Tropsch (F-T) technology have attracted more attention. A review of proposed and existing F-T projects along with a discussion of the economics of the projects has recently been published (Oil and Gas J., Sep. 21 and Sep. 28, 1998). In this technology, natural gas is first converted to “syngas,” which is a mixture of carbon monoxide and hydrogen, and the syngas is then converted to liquid paraffinic and olefinic hydrocarbons of varying chain lengths.
The conversion of natural gas to unsaturated hydrocarbons and hydrogen by subjecting the hydrocarbons in natural gas to high temperatures produced by electromagnetic radiation or electrical discharges has been extensively studied. U.S. Pat. No. 5,277,773 (Exxon Research & Eng. Co.) discloses a conversion process that treats methane and hydrocarbons with microwave radiation so as to produce an electric discharge in an electromagnetic field. U.S. Pat. No. 5,131,993 (The Univ. of Conn.) discloses a method for cracking a hydrocarbon material in the presence of microwave discharge plasma and a carrier gas, such as oxygen, hydrogen, and nitrogen and, generally, a catalyst. Expired U.S. Pat. No. 3,389,189 (Westinghouse Electric Corp.) is an example relating to the production of acetylene by an electric arc.
The traditional methods of converting lower molecular weight carbon-containing molecules to higher molecular weights are numerous. There are many patents that teach reactor designs with the purpose of converting hydrocarbon containing gases to ethylene, acetylene, or syngas. The most prevalent methods involve oxidative coupling, partial oxidation, or pyrolysis. Each method has its own benefits and its own challenges.
Oxidative coupling is a technique wherein a lighter hydrocarbon is passed through a reaction bed containing a catalyst that encourages partial oxidation of the hydrocarbon. The primary advantage of oxidative coupling is that relatively mild conditions of temperature and pressure are required. Another real advantage of oxidative coupling is that liquid hydrocarbons (and other liquids) can be formed in substantial quantity. The distinguishing disadvantage of oxidative coupling is the necessity for a solid phase catalyst, which has a short useful life and must be regenerated often. U.S. Pat. No. 4,704,493 (Chevron Corp.) discloses the use of Group IIA metal oxides on various supports to convert methane into light aromatic compounds and light hydrocarbons. Although methane conversions of up to 40% are reported, there is a strong correlation between increased conversion and increased tar and coke production. U.S. Pat. No. 4,705,908 (Gondouin) teaches the conversion of natural gas containing components of C1 through C4-C5+ hydrocarbons and hydrogen by first splitting the stream of natural gas into a C1 -C2 portion and a heavier portion, and then reacting these streams separately using a single non-silica based catalyst that includes mixed oxides. The reactions are performed at different temperatures and residence times. Disadvantages of this process include expected low conversion, excessive recycling of gases, continuous movement, and regeneration of the solid catalyst. U.S. Pat. No. 5,012,028 (The Standard Oil Co.) presents a process whereby natural gas is separated into methane and C2+ hydrocarbons and other gases, and the methane is introduced along with oxygen to a reactor operated to perform oxidative coupling. The products of oxidative coupling are then combined with the other gases and non-methane hydrocarbons in a pyrolysis reactor. A quench step and a product recovery step follow. A disadvantage of this process is that the overall conversion to liquids is low (<10%). U.S. Pat. No. 5,288,935 (Institut Francais du Petrole) teaches separating natural gas into methane and other gases rich in C2+. The methane is subjected to oxidative coupling. The C2+ fraction is fed to the reactor before all of the oxygen is consumed. The product from this reactor is conveyed to an aromatization reactor, containing a catalyst comprising an MFI zeolite containing gallium. Conversion to heavier components is about 10% to 15%. U.S. Pat. No. 6,518,476 (Union Carbide Chem. & Plas. Tech. Corp.) teaches effective oxidative dehydrogenation of natural gas at elevated pressure, generally between 50 psi and 400 psi (about 340-2800 kPa) and below 600° C., using a rare earth oxycarbonate catalyst. The olefin yield is increased through recycling of the non-olefin containing product. The olefin is removed using silver ion-containing complexation agents or solutions. Conversions are generally on the order of 20% but can be as high as 40%, depending upon the method of operation of the reactor. Selectivity declines with increased conversion. U.S. Pat. No. 6,566,573 (Dow Global Tech., Inc.) teaches conversion of paraffinic hydrocarbons with two or more carbon atoms to olefins in the presence of oxygen, hydrogen, and a supported platinum catalyst. It is recognized that preheating of the feedstreams reduces the required flow of oxygen, with a resulting reduction in oxygen-containing byproducts such as CO and CO2. Conversion of ethane to ethylene is about 55%, while acetylene production is less than 1%.
Non-catalytic partial oxidation is widely practiced because the technique is simpler as there is no catalyst to regenerate. Products generally include only gas phase components, which will generally include ethylene, carbon monoxide, carbon dioxide, and acetylene. There are many reactor designs and methods for partial oxidation. U.S. Pat. No. 4,575,383 (Atlantic Richfield Co.) discloses a unique reactor design, namely a reciprocating piston engine. Conversion of methane to ethylene and acetylene is less than 1% however, which is very low. U.S. Pat. Nos. 4,599,479 and 4,655,904 (Mitsubishi Jukogyo Kabushiki Kaisha) teach a technique to increase the yield of BTX (benzene/toluene/xylene) compounds in one reactor by first burning a hydrocarbon with less-than-stoichiometric oxygen to make a hot gas containing steam and hydrogen, and then feeding methane and hydrogen to the hot gas formed, followed by a quench. More BTX can be made by feeding an intermediate stream containing liquid hydrocarbons which have a normal boiling point above 350° C. It is taught that the methane to hydrogen ratio is very important, as the hydrogen tends to consume olefins generated while at the same time generating methyl radicals that lead to the formation of heavier hydrocarbon species. The reaction time of 15 milliseconds is relatively long. U.S. Pat. No. 5,068,486 (Mobil Oil Corp.) reveals a partial oxidation process that operates at very high pressure (20-100 atm), necessitating very high compression costs. The conversion of methane, which is the hydrocarbon feed, is reported as 12.6%, with hydrocarbon selectivity of 32%. The overall conversion of methane to ethylene, acetylene, and propane were 1.4%, 0.4% and 0.1%, respectively. U.S. Pat. Nos. 5,886,056 and 5,935,489 (Exxon Res. and Eng. Co.) teach a multi-nozzle design for feeding a partial oxidation reactor. The multiple nozzles allow introduction of a pre-mix of oxidant and fuel at the burner face so that these gases are premixed and of uniform composition. Alternatively, the plurality of injection nozzles allows one to feed different pre-mix compositions to the partial oxidation reactor burner face, for example allowing one nozzle to act as a pilot due to a higher than average oxygen feed concentration, and those nozzles on the periphery to have a greater hydrocarbon concentration resulting in a lower temperature. A major disadvantage of such a design is that the control and operation of multiple feeds increases the probability of failure or shutdown of the reactor and also increases the cost of building the reactor. U.S. Pat. No. 6,365,792 (BASF AG) teaches that operation of a partial oxidation cracker at less than 1400° C. but for longer residence times provides similar acetylene conversion but at reduced energy costs and with less solid carbon being formed.
Pyrolysis of hydrocarbons generally requires higher temperatures than the other techniques because there are normally no oxidative or catalytic species present to facilitate dehydrogenation of the hydrocarbon. As in oxidation processes, the products are generally limited to gas phase components.
There are many ways to propagate pyrolysis reactions and some are described here. Expired U.S. Pat. No. 3,663,394 (The Dow Chem. Co.) claims use of a microwave generated plasma for converting methane and ethane to acetylene. Although conversions ranged up to 98% with about 50% acetylene being formed, the process performed best at pressures below 40 torr and especially at 10 torr, which would be difficult to achieve economically at industrial scale. Expired U.S. Pat. No. 3,697,612 (Westinghouse Elec. Corp.) describes an arc heater of complex design that can convert methane to higher hydrocarbons, wherein the conversion is about 40%. Of the total converted, acetylene accounted for 74% of the product. The energy required to create a pound of acetylene was more than 5 kilowatts, which is comparable to other methods for making acetylene using electrical discharge. Expired U.S. Pat. No. 3,703,460 (U.S. Atomic Energy Commission) teaches that ethylene and ethane can be made in an induced electric discharge plasma reactor. The process operates at atmospheric pressure or below and provides less than 6% conversion of the feed methane. A disadvantage of the process is the need for vacuum pumps, which are expensive to operate. U.S. Pat. No. 4,256,565 (Rockwell Int'l. Corp.) discloses a method to produce high yields of olefins from heavy hydrocarbon feedstock by commingling a stream of hot hydrogen and water vapor with a spray of liquefied heavy hydrocarbon consisting preferentially of asphalts and heavy gas oils. Yields of olefins are strongly dependent upon rapid heating and then cooling of the fine spray droplets, to initiate and then quench the reactions. U.S. Pat. No. 4,288,408 (L.A. Daly Co.) teaches that for cracking of heavy hydrocarbons, which tend to coke heavily, injection of an inert gas such as nitrogen or CO2 just downstream of the liquid feed atomizers will decrease accumulation and formation of coke on the walls of the reactor and downstream in the gas cooler. U.S. Pat. No. 4,704,496 (The Standard Oil Co.) relates to the use of nitrogen and sulfur oxides as reaction initiators for pyrolysis of light hydrocarbons in reactors such as tubular heaters. Conversion of methane is reportedly as high as 18.5%, with selectivity to liquids as high as 57.8% and selectivity to acetylene as high as 18.7%. No mention of liquid composition is provided, so it is reasonable to suspect that some heteroatom incorporation into the liquid molecules occurs. U.S. Pat. No. 4,727,207 (Standard Oil Co.) teaches that the addition of minor amounts of carbon dioxide to methane or natural gas will assist in the conversion of the methane or natural gas to higher molecular weight hydrocarbons as well as reduce the amount of tars and coke formed. The examples were run at 600° C., which is a relatively low temperature for pyrolysis of methane, and the reported conversions were generally low (about 20% or less). A drawback of this technique is that the addition of CO2 adds another component that must then be removed from the product, which increases both gas scrubbing costs and transmission equipment size.
U.S. Pat. No. 5,749,937 (Lockheed Idaho Tech. Co.) discloses that acetylene can be made from methane using a hydrogen torch with a rapid quench, with conversions of methane to acetylene reportedly 70% to 85% and the balance being carbon black. U.S. Pat. No. 5,938,975 (Ennis et al.) discloses the use of a rocket engine of variable length for pyrolysis of various feeds including hydrocarbons. Various combinations of turbines are disclosed for generating power and compressing gas, purportedly allowing a wide range of operating conditions, including pressure. An obvious drawback of such a rocket powered series of reactors is the complexity of the resulting design. U.S. Patent No. Application Publication No. 20030021746, U.S. Pat. Nos. RE37,853E and 6,187,226 (Bechtel BWXT Idaho, LLC), and 5,935,293 (Lockheed Martin Idaho Tech. Co.) all teach a method to make essentially pure acetylene from methane via a plasma torch fueled by hydrogen. The disclosed design employs very short residence times, very high temperatures, and rapid expansion through specially designed nozzles to cool and quench the acetylene production reaction before carbon particles are produced. The disclosed technique purportedly enables non-equilibrium operation, or kinetic control, of the reactor such that up to 70% to 85% of the product is acetylene. Approximately 10% of the product is carbon. A drawback of this process is that high purity hydrogen feed is required to generate the plasma used for heating the hydrocarbon stream.
Interesting combinations of processes have also been developed. For example, U.S. Pat. No. 4,134,740 (Texaco Inc.) uses carbon recovered from the non-catalytic partial oxidation reaction of naphtha as a fuel component. A complex carbon recovery process is described wherein the reactor effluent is washed and cooled with water, the carbon is extracted with liquid hydrocarbon and stripped with steam, and then added to an oil to form a slurry that is fed back to the partial oxidation reactor. This process does not appear to be applicable to the partial oxidation of gas-phase hydrocarbons, however. The handling and conveying of slurries of carbon, which clogs pipes and nozzles, is a further drawback. U.S. Pat. No. 4,184,322 (Texaco Inc.) discusses methods for power recovery from the outlet stream of a partial oxidation cracker. The methods suggested include: 1) heat recovery steam generation with the high temperature effluent gas, 2) driving turbines with the effluent gas to create power, 3) directly or indirectly preheating the partial oxidation reactor feeds using the heat of the effluent, and 4) generating steam in the partial oxidation gas generator to operate compressors. Integration of these methods can be difficult in practice. For example, when preheating feed streams depends on the downstream temperature and effluent composition, there will be periods when the operation is non-constant and the product composition is not stable. However, no external devices are disclosed to assist in the start-up or trim of the operation to achieve or maintain stable operation and product quality. U.S. Pat. No. 4,513,164 (Olah) discloses a process combining thermal cracking with chemical condensation, wherein methane is first cracked to form acetylene or ethylene, which is then reacted with more methane over a superacid catalyst, such as tantalum pentafluoride. Products are said to consist principally of liquid alkanes. U.S. Pat. No. 4,754,091 (Amoco Corp.) combines oxidative coupling of methane to form ethane and ethylene with catalytic aromatization of the ethylene. The ethane formed and some unreacted methane is recycled to the reactor. Recycle of the complete methane stream did not provide the best results. The preferred lead oxide catalyst achieved its best selectivity with a silica support, and its best activity with an alpha alumina support. Residual unsaturated compounds in the recycle gas were said to be deleterious in the oxidative coupling reaction. It is also taught that certain acid catalysts were able to remove ethylene and higher unsaturates from a dilute methane stream, without oligomerization, under conditions of low pressure and concentration. Expired U.S. Pat. No. 4,822,940 (The Standard Oil Co.) discloses the conversion of a feedstock containing hydrogen, ethylene, and acetylene to a product with a substantial liquid content in a conventional non-catalytic pyrolysis reactor, when the contents are maintained at about 800° to 900° C. for about 200 to 350 milliseconds. One of the reported examples shows 30% ethylene conversion and 70% acetylene conversion to liquids, with more than 80% selectivity to liquids.
U.S. Pat. No. 5,012,028 (The Standard Oil Co.) teaches the combination of oxidative coupling and pyrolysis to reduce external energy input. Oxidative coupling is used to form an intermediate, principally ethylene and ethane, which is an exothermic process. The product of the oxidative coupling reaction is converted to heavier hydrocarbons, which is endothermic, in a pyrolysis reactor. Pyrolysis of C2+ hydrocarbons to liquids does not require as high a temperature as does the pyrolysis of methane, therefore the required energy input is reduced. Because both process steps occur at temperatures below 1200° C., equipment can be readily designed to transfer heat between the processes for heat integration. A major drawback of this combination of technologies however, is controlling the composition of the intermediate because residence times are less than ½ second in both systems. Feed or control fluctuations could easily result in loss of operation and heat transfer between the units. If the units are closely coupled, such a loss of heat transfer could easily result in reactor damage. U.S. Pat. No. 5,254,781 (Amoco Corp.) discloses oxidative coupling and subsequent cracking, wherein the oxygen is obtained cryogenically from air and the products, principally C2's and C3's, are liquefied cryogenically. Effective heat integration between the exothermic oxidative coupling process step and the endothermic cracking process step is also said to be obtained. U.S. Pat. No. 6,090,977 (BASF AG) uses a hydrocarbon diluent, such as methane, to control the reaction of a different, more easily oxidized hydrocarbon, such as propylene. The more easily oxidized hydrocarbon is converted by heterogeneously catalyzed gas phase partial oxidation. After the partial oxidation reaction, combustion of the effluent gas is used to generate heat. An advantage of a hydrocarbon diluent is that it can absorb excess free radicals and thereby prevent run-away reaction conditions caused by the presence of excess oxygen. The hydrocarbon also increases the heating value of the waste gas, thus its value as a fuel. Of course, this technique cannot be utilized when the reaction conditions are such that methane reacts and/or is the predominant reactant. U.S. Pat. No. 6,596,912 (The Texas A&M Univ. System) employs a recycle system with a high recycle ratio of (8.6:1) to achieve a high conversion of methane to C4 and heavier products. The initial process employs an oxidative coupling catalyst to produce primarily ethylene, and a subsequent process step using an acid catalyst such as ZSM-5 to oligomerize the ethylene. A drawback of this relatively high recycle ratio is that larger compressors and reactors are required to produce the final product.
To produce liquids after cracking, oligomerization of the unsaturated cracked hydrocarbons can produce a desirable liquid composition. U.S. Pat. No. 5,118,893 (Board of Regents, The Univ. of Texas System) for example, discloses a high conversion of acetylene directly to other hydrocarbons using a nickel or cobalt modified ZSM catalyst. Conversions of 100% are reported for up to 8 hours of operation. Conversion to liquid products after several hours of operation appears to stabilize between 10 and 20%. Data for longer times are not given for the modified catalysts. U.S. Pat. No. 4,424,401 (The Broken Hill Prop. Co. Ltd.; Commonwealth Scientific; and Industrial Res. Org.) teaches use of a ZSM-5 zeolite with a minimum ratio of silica to alumina of 800:1 to convert acetylene and hydrogen to liquid hydrocarbons. Many oligomerization catalysts are highly sensitive to the presence of water. However, U.S. Pat. No. 4,982,032 (Amoco Corp.) teaches that acetylene can be oligomerized while water is in significant abundance by HAMS-1B crystalline borosilicate modified molecular sieve promoted by zinc oxide. The catalyst is also said to be tolerant of CO, CO2, O2 and alcohols. Although the reported conversions are high, the optimum selectivity to organic liquids is reported to be only about 73%. The use of gas streams low in acetylene content resulted in much lower acetylene conversion.
Following cracking, some unsaturated compounds are desirably converted to hydrogenated species. The hydrogenation of unsaturated compounds is known in the art. For example, U.S. Pat. No. 5,981,818 (Stone & Webster Eng. Corp.) teaches the production of olefin feedstocks, including ethylene and propylene, from cracked gases. U.S. Pat. No. 5,414,170 (Stone & Webster Eng. Corp.) discloses a mixed-phase hydrogenation process at very high pressure. A drawback of this technique is that the concentration of acetylene must be low to enable the proper control of temperature in the hydrogenation step. U.S. Pat. No. 4,705,906 (The British Petroleum Co.) teaches hydrogenation of acetylene to form ethylene in the gas phase using a zinc oxide or sulphide catalyst. Conversions up to 100% and selectivities to ethylene up to 79% were reported.
Separation of the products of cracking is often desirable when a specific component has particular value. For example, separation of acetylene from ethylene is beneficial when the ethylene is to be used in making polyethylene. U.S. Pat. No. 4,336,045 (Union Carbide Corp.) proposes the use of liquid hydrocarbons to separate acetylene from ethylene, using a light hydrocarbon at temperatures of below −70° C. and elevated pressure.
The cogeneration of electrical power can substantially improve the economics of cracking processes. For example, U.S. Pat. No. 4,309,359 (Imperial Chem. Ind. Ltd.) describes the use of a catalyst to convert a gas stream containing hydrogen and carbon monoxide to methanol, whereby some of the gas is used to create energy via reaction in a fuel cell.
Chemical production prior to the complete separation of the products of the cracking reaction can also be used to reduce the cost of purification. U.S. Pat. No. 4,014,947 (Volodin et al.) describes a process for the pyrolysis of hydrogen and methane with conversion of the produced acetylene and ethylene to vinyl chloride. The acetylene and ethylene are reacted with chlorine or hydrogen chloride, during the pyrolysis formation of the unsaturated hydrocarbons, and rapidly quenched with a liquid hydrocarbon.
U.S. Pat. Nos. 6,130,260 and 6,602,920 (The Texas A&M Univ. Systems) and U.S. Pat. No. 6,323,247 (Hall et al.) describe a method in which methane is converted to hydrogen and acetylene at temperature, quenched, and catalytically converted to, inter alia, pentane. While an advance over conventional art processes, the method disclosed still suffers from a number of drawbacks with respect to the preferred embodiments of the process of the present invention, as will be further described herein. In particular, the production and integration of carbon monoxide and carbon dioxide within the process is not contemplated by the reference. Carbon monoxide is produced in preferred embodiments of the present invention that include a partial oxidation step, and it provides additional value to the inventive processes as both a downstream feedstock and a fuel. Carbon dioxide that can be used to reduce the carbon formation in process equipment and increase the overall process yield is also produced in preferred embodiments of the present invention that include direct heating.
Further advantages are provided by the employment of the various separation processes described in preferred embodiments of the present invention. For example, preferred embodiments of the present invention provide for the separation of acetylene from other gas components prior to hydrogenation, with corresponding reductions in the quantity of gas that must be treated in the hydrogenation steps. Improvements in catalyst life may also be expected therefrom. Ethylene management in accordance with preferred embodiments of the present invention provides additional advantages, as illustrated by inventive preferred embodiments comprising removal of ethylene from acetylene-deprived streams, with their subsequent combination with ethylene-rich hydrogenator product streams. In some preferred embodiments of the present invention, fractionation of the natural gas feed prior to conversion steps allows different reaction conditions for the various fractions, thus improving the performance of the overall process and optimization of the product mix.
Additional advantages are provided by the unit operations uniquely employed in accordance with preferred embodiments of the processes of the present invention. Direct heat exchange, but one such example, is utilized to enhance conversion and reduce carbon formation in certain preferred embodiments of the present invention by placing the heating medium in direct contact with the reactant gas, thus enabling chemical reactions and equilibria that would not otherwise obtain. Similarly, the above-mentioned conventional processes do not disclose the recycle of gas components other than hydrogen to the combustor for the indirect transfer of heat, or for combination with the incoming natural gas feed stream. Preferred embodiments of the present invention however, provide for the separation of non-hydrogen components upstream of the hydrogenator and downstream of the catalytic reactor with recycle for improving the acetylene yield, with the further option of recycle to the combustion stage, if the heating value of the stream provides an economic advantage.
Numerous methods for cracking hydrocarbons, particularly natural gas and methane, are known in the art Likewise, many methods have been developed for separation of the products from cracking reactions, and many designs have been disclosed for producing ethylene and acetylene from cracking processes. However, no economical and integrated method is presently known in the art for the conversion of methane and natural gas to ethylene, hydrocarbon liquids, and other valuable final products, through the intermediate manufacture of acetylene, such that the final products can be either transported efficiently from remote areas to market areas (or used at the point of manufacture).
Although the prior art discloses a broad range of methods for forming acetylene or ethylene from natural gas, an energy-efficient process for converting natural gas to liquids that can be transported efficiently from remote areas to market areas has not previously been available. A way of overcoming these problems is needed so that production of transportable liquids from natural gas is practical for commercial industrial-scale applications. Accordingly, research has focused on developing new processes that can reduce or eliminate the problems associated with the prior art methods. The processes of the present invention in their various preferred embodiments are believed to both overcome the drawbacks of the prior art and provide a substantial advancement in the art relating to the conversion of natural gas to transportable hydrocarbon liquids. The present invention has been developed with these considerations in mind and is believed to be an improvement over the methods of the prior art.